The combined cycle power plant is a power generation system which has combined gas and steam turbine plants. In this system, the gas turbine takes charge of a higher-temperature range of the thermal energy, while the steam turbine takes charge of the remaining lower-temperature range, thereby efficiently recovering and using the thermal energy. This is currently the most common power generation system.
In order to improve the efficiency, the development of the combined cycle power plant has been focused on how high the higher-temperature range can be set.
To realize a higher-temperature range, a cooling system must be provided in consideration of the heat resistance of the turbine structure. Conventionally, air is used as a cooling medium in such a cooling system.
However, as long as air is used as the cooling medium, even if a desirable higher-temperature range can be achieved, the plant will inevitably suffer from (i) loss of power necessary for boosting the air (which was used for cooling) up to a specified pressure by using an internal air compressor, and (ii) lowering of the average gas temperature and thus the energy of the gas because the air used for cooling target portions is finally made flow through the passage (in the turbine) together with the high-temperature gas. As a result of the above effects (i) and (ii), it is very difficult to further improve the thermal efficiency.
In order to solve the above problem and to further improve the efficiency, another type of combined cycle power plant has been proposed, in which the steam is used as the cooling medium of the gas turbine instead of air as mentioned above. Japanese Unexamined Patent Application, First Publication, No. Hei 5-163960 discloses an example thereof. The general structure of the disclosed power plant is shown in FIG. 2.
That is, the combined cycle power plant 10 comprises (i) gas turbine plant 11 mainly including gas turbine 13, air compressor 18, and combustor 19, (ii) exhaust heat recovery boiler 14, mainly comprising high-pressure drum 20, medium-pressure drum 21, and low-pressure drum 22, which uses the exhaust gas from the gas turbine plant 11 as a heat source, and (iii) steam turbine plant 12 mainly comprising high-pressure turbine 15a, medium-pressure turbine 15b, and low-pressure turbine 15c to which the steam is supplied from the exhaust heat recovery boiler 14.
The cooling system employed here is steam cooling system 50 in which the medium-pressure steam from the medium-pressure drum 21 of the exhaust heat recovery boiler 14 is introduced as the cooling steam via steam supply path 51 into steam cooling section 52 provided in a higher-temperature portion to be cooled in gas turbine 13. That is, the above higher-temperature portion is cooled, and thus the above cooling steam is heated and obtains energy. The cooling steam is then supplied via steam recovery system 53 to the medium-pressure turbine 15b of the steam turbine plant 12. That is, the steam can be efficiently recovered.
Here, steam section 60 is a backup section, and the backup steam can be supplied from the high-pressure drum 20 of the exhaust heat recovery boiler 14 via high-pressure steam line 42. This backup section is used immediately after the gas turbine 13 is activated.
As explained above, the conventional system uses the medium-pressure steam obtained from the medium-pressure drum 21 as the cooling steam; thus, the temperature at the inlet of the gas turbine is further increased or the target high-temperature portion or area to be cooled in the gas turbine is extended. That is, the extended high-temperature portion to be cooled may include moving blades, stationary blades, and further the circular portion of the turbine. As the area to be cooled extends, the heat load of the area increases and the cooling capability of the medium-pressure steam decreases and becomes insufficient because the amount of the steam generated by the exhaust heat recovery boiler is limited. Accordingly, the original objective to sufficiently and reliably cool the target high-temperature portion cannot be achieved.
It is an object of the present invention to solve the above problems in the conventional system, and to provide a combined cycle power plant, whose target high-temperature portion in the gas turbine can be reliably and sufficiently cooled, and in which the heat generated via the cooling can be reliably recovered, thereby improving the efficiency.